There is an increasing demand for so-called liquid crystal lenses whose focal length can be varied in accordance with voltage applied to the liquid crystal element. In an autofocus mechanism or optical variable magnification mechanism used in a conventional digital camera or the like, the provision of a mechanical system for moving a lens or lenses is essential, which not only requires space for mounting, but also increases the cost. On the other hand, in the case of liquid crystal lenses, since no moving parts are required, a space-saving, low-cost autofocus mechanism or optical variable magnification mechanism can be achieved.
For example, a liquid crystal lens is known, which is formed by placing a liquid crystal between two glass substrates and cutting out one of the glass substrates into the shape of a lens, and in which voltage is applied to the liquid crystal thereby causing the effective refractive index of the liquid crystal to change, thus changing the focal length by utilizing the refractive index difference between the glass and the liquid crystal (for example, refer to patent document 1). The liquid crystal lens thus formed by cutting out glass into the shape of a lens can be constructed to have an ideal refractive index profile.
However, cutting out a glass substrate into the shape of a lens is a time-consuming and cost-intensive job, and it has not been possible to provide an inexpensive, high-performance liquid crystal lens that is capable of varying focus length and is relatively free from aberrations.
Further, an autofocus alignment lens system is described in which the focal length is finely varied using a variable focal length liquid crystal lens (for example, refer to patent document 2).
However, the specific structure of the variable focal length liquid crystal lens has not been shown in the prior art. Furthermore, in the prior art, the variable focal length liquid crystal lens has only been used for fine adjustment of the focal length, and the system has been equipped with a separate focusing lens for varying the focal length and a moving mechanism for the same.
There is also a known liquid crystal panel, which is formed by placing a liquid crystal between two transparent substrates with an electrode pattern of a plurality of concentric circles and a counter electrode interposed therebetween, and in which voltage is applied to the electrode pattern to form a Fresnel zone plate utilizing the liquid crystal, and the pattern of the Fresnel zone plate is varied to perform spatial frequency modulation thereby causing the focal length to change (for example, refer to patent document 3).
However, to vary the focal length, the electrode pattern itself has had to be changed, and this has required a special technique for changing the electrode pattern in accordance with the situation. That is, the liquid crystal panel described in patent document 3 was intended for a liquid crystal optical element that functions as a gradient-index lens for varying the focal length by using a fixed electrode pattern.
It is also known to construct a focusing system using a liquid crystal in which a liquid crystal is placed between two parallel planar substrates by interposing therebetween a plurality of electrodes arranged in equally spaced concentric circles and a counter electrode, with a resistor interposed between each electrode, and in which prescribed voltage is applied to each particular electrode by resistive dividing, thereby causing the effective refractive index of the liquid crystal between the electrode and the counter electrode to change and thus causing the focal length to change (for example, refer to patent document 4).
FIG. 17(a) shows an electrode pattern 12 such as described in patent document 4. The electrode pattern 12 has four electrodes 12-1 to 12-4 arranged in equally spaced concentric circles. FIG. 17(b) shows isoelectric potentials V1 to V4 applied to the respective electrodes in the electrode pattern by resistive dividing.
FIG. 18(a) shows the applied voltages shown in FIG. 17(b). FIG. 18(b) shows effective refractive indices n1 to n4 generated according to the respective electrodes when potentials such as shown in FIG. 18(a) are applied. When the isoelectric potentials V1 to V4 formed by resistive dividing are applied to the respective electrodes 12-1 to 12-4 in the electrode pattern, the effective refractive indices n1 to n4 changing in equal steps occur as shown in FIG. 18(b). However, to obtain an ideal refractive index profile (1800 in FIG. 18(b)) such as shown in FIG. 15 in patent document 4, effective refractive indices n1 to n4 that the respective electrodes 12-1 to 12-4 generate must be individually fine-adjusted as shown by arrows in FIG. 18(b).
In this way, in the electrode pattern 12 arranged in equally spaced concentric circles, it is extremely difficult to set the voltage by using a resistive dividing method and to fine-adjust the applied voltage so as to form the refractive index gradient as designed. Furthermore, with the resistive dividing method, since the voltage to be applied to each individual electrode cannot be finely adjusted, it has not been possible to form a liquid crystal lens having an ideal refractive index profile.
Patent document 1: Japanese Unexamined Patent Publication No. S62-56918 (FIG. 2)
Patent document 2: Japanese Unexamined Patent Publication No. S62-36632 (page 3, FIG. 1)
Patent document 3: Japanese Patent No. 2651148 (page 1, FIG. 1)
Patent document 4: Japanese Patent No. 3047082 (page 4, FIGS. 1 and 3)